1. Field of the Invention
The present invention relates to an organic electroluminescent element (organic EL element) and a production method thereof. More specifically, the invention relates to an organic EL element used for a display, etc., and to a production method thereof.
2. Background of the Invention
Recently, with the highly informational increase, a need of a low electric power consuming and light-weight display element thinner than then CRT for a full color flat display has been increased. As the display element of this kind, a non-spontaneous light-emitting type liquid crystal display (LCD), a spontaneous light-emitting type plasma display (PDP), an EL display, etc., are known.
In the above-described display elements, the EL display is classified into two types of (1) an intrinsic EL element exciting a material (light emitting material) constituting a light emitting layer by the local transfer of electrons or holes in the light emitting layer and emitting light by an alternating current electric field and (2) a charge-injection type EL element exciting a light emitting material by the injection of electrons and holes from an electrode and the recombination of them in a light emitting layer and emitting light by a direct current electric field, from the differences in the light-emitting exciting mechanisms and the constituting materials.
For the intrinsic EL element of above-described (1), a light emitting material made of an inorganic material is generally used and for the EL light emitting element of an injection type of above-described (2), a light emitting material made of an organic material is generally used. That is, there are relations that intrinsic EL element=inorganic EL element and charge injection type EL element=organic EL element.
In these elements, a display using, particularly, the organic EL element is being watched with extremely interest because the display has the features that the display is a spontaneous light emitting type, a low electric power consumption can be realized, the light-emitting color is various, etc.
Hitherto, as an example of the construction of the organic EL element, an element having a three-layer structure shown in FIG. 13 of the accompanying drawings is well known (xe2x80x9cAppl. Phys. Lett.xe2x80x9d, 56(9), Feb. 26, 1990). The organic EL element of prior art shown in FIG. 13 has the construction that on a transparent substrate 1 such as a glass are successively laminated an anode 2 made of an electrode material having a large work function, such as indium-tin oxide (ITO), a hole transporting layer 3 containing a hole transporting material 13, a light emitting layer 52, an electron transporting layer 7 containing an electron transporting material 17, and a cathode 8 made of an electrode material having a small work function, such as Mg/Ag. In the organic EL element, a voltage is applied between the electrodes 2 and 8 to inject electrons and holes in the light emitting layer 52, and by releasing the recombination energy of an electron-hole pair formed in the light emitting layer as a fluorescence or a phosphorescence, a light is emitted.
As a method of producing such an organic EL element, a dry process such as a vacuum vapor-deposition method of forming film on a substrate by heat-sublimating organic materials constituting the organic EL element in vacuum and a wet process such as a Langmuir-Blodgett method (LB method) of spreading an organic material on a liquid surface as a thin film and transferring the film onto a substrate and a spin-coating method of preparing a solution by dissolving an organic material in a solvent and spin-coating the solution by dropping the solution onto a substrate, etc., are known.
However, because a coloring material used for each of the organic layers (a hole transporting layer, a light emitting layer, and an electron transporting layer) in the organic El element has the absorption ends from a visible light region to a ultraviolet region, the band gap is 1.5 eV or higher and there scarcely exists a carrier in the inside of each organic layer at room temperature. Also, the mobility of the carrier moving in the coloring material is generally small.
By these reasons, the electric resistances of the organic layers are very high. The high-resistance characteristics of the organic layers cause voltage lowering of the element and generation of a Joule""s heat and further by the causes, there occurs a problem that the light emitting efficiency and the life are lowered.
As a method of solving the problem, a method of improving the electric conductivities of the organic layers by increasing the concentration of the carrier in the inside of the hole transporting layer and the electron transporting layer in a thermal equilibrium state by doping the hole transporting layer with an acceptor 23 and the electron transporting layer with a donor 27 is proposed as shown in Japanese Unexamined Patent Publication No. HEI 4 (1992)-2907076 (FIG. 14).
However, in the above-described method, the electric conductivities of the organic layers can be improved but the carrier cannot be sufficiently confined. As the result thereof, there are problems that the light emitting efficiency is lowered and with the increase of the leaked electric current by a reverse bias, the rectification characteristics become worse.
As described above, the organic EL element strongly suggests the possibility as a full color flat panel display element of the next generation but in the production of the practical element, there are problems which must be solved.
The present invention has been made for solving the above-described problems and an object of this invention is to provide an organic electroluminescent element of the construction having lowered resistances of organic layers, having a high light emitting efficiency, and being excellent in electrooptical characteristics and also to provide a production method of the organic electroluminescent element.
That is, according to an aspect of this invention, there is provided an organic electroluminescent element comprising at least a light emitting layer containing an organic light emitting material placed between an anode and a cathode, wherein the element has, between the anode and the light emitting layer, at least a hole transporting layer containing a hole transporting material and an acceptor, and an electron injection restraining layer restraining the injection of electrons from the light emitting layer into the hole transporting layer, from the anode side, and/or, between the light emitting layer and the cathode, at least an electron transporting layer containing an electron transporting material and a donor, and a hole injection restraining layer restraining the injection of holes from the light emitting layer into the electron transporting layer, from the cathode side.
Furthermore, according to another aspect of this invention, there is provided a method of producing the above-described organic electroluminescent element comprising forming each of the anode, the hole transporting layer, the electron injection restraining layer, the light emitting layer, the hole injection restraining layer, the electron transporting layer, and the cathode by a vacuum film-forming method.
These and other objects of the present application will become more readily apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.